Vertical thin film transistor selection devices and methods of fabrication

ABSTRACT

Three-dimensional (3D) non-volatile memory arrays having a vertically-oriented thin film transistor (TFT) select device and method of fabricating such a memory are described. The vertically-oriented TFT may be used as a vertical bit line selection device to couple a global bit line to a vertical bit line. A select device pillar includes a body and upper and lower source/drain regions. At least one gate is separated horizontally from the select device pillar by a gate dielectric. Beneath each gate, a single gap fill dielectric layer extends vertically from a lower surface of the gate, at least partially separating the gate from the underlying global bit line. Between horizontally adjacent pillars, this same dielectric layer extends from its same lower level beneath the gates vertically to a level of the upper source/drain region.

BACKGROUND OF THE INVENTION

The present disclosure relates to technology for non-volatile storage.

One example of non-volatile memory uses variable resistance memory elements that may be set to either low or high resistance states. Upon application of sufficient voltage, current, or other stimulus, the variable resistance memory element switches to a stable low-resistance state, which is sometimes referred to as SETTING the device. This resistivity-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistivity-switching material to a stable high-resistance state, which is sometimes referred to as RESETTING the device. This conversion can be repeated many times.

The variable resistance memory elements may be in a high resistance state when first manufactured. This may be referred to as the “virgin state.” In the virgin state, the resistance could be even higher than for the RESET state. The term “FORMING” is sometimes used to describe putting the variable resistance memory elements into a lower resistance state for the first time. For some memory elements, the FORMING operation requires a higher voltage than the SET and/or RESET operations.

3D memory arrays having variable resistance memory elements have been proposed. In one possible architecture, word lines extend horizontally and bit lines extend vertically. There a multiple levels of the word lines, hence multiple levels of memory elements. Each memory element is located between one of the vertical bit lines and one of the horizontal word lines. During operation, some of the memory cells are selected for the SET, RESET, or FORM operation, while others are unselected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit for a portion of an example three-dimensional array of variable resistance memory elements.

FIG. 2 is an equivalent circuit for a portion of another example three-dimensional array of variable resistance memory elements.

FIG. 3 is a schematic of a portion of a memory system, depicting vertical bit lines, vertically oriented select devices above the substrate and row select line drivers in the substrate.

FIG. 4 is a schematic block diagram of a re-programmable non-volatile memory system which can utilize the memory arrays of FIGS. 1-3, and which indicates connection of the memory system with a host system.

FIG. 5 is an isometric view of a portion of the three-dimensional array shown in FIG. 2 according to a one example of an implementation.

FIG. 6 is a plan view that depicts a vertical bit line, a vertically oriented select device and a global bit line.

FIG. 7 depicts the structure of two vertical TFT selection devices in accordance with one embodiment.

FIG. 8 is a cross-sectional view of a memory structure using one embodiment of a vertically oriented TFT selection device and the memory structure of FIG. 5.

FIG. 9 is a flowchart describing a method of fabricating a pillar select layer including vertical TFT selection devices in one embodiment.

FIGS. 10a-10j are perspective and cross-sectional views according to a fabrication process in one embodiment.

DETAILED DESCRIPTION

The disclosed technology is directed to a three-dimensional (3D) non-volatile memory array having a vertically-oriented thin film transistor (TFT) select device and method of fabricating such a memory. The vertically-oriented TFT, or more simply vertical TFT, may be used as a vertical bit line selection device in one embodiment, coupling a global bit line to a local vertical bit line. A select device pillar includes a body and upper and lower source/drain regions. At least one gate is separated horizontally from the select device pillar by a gate dielectric. Beneath each gate, a single gap fill dielectric layer extends vertically from a lower surface of the gate. The gap fill dielectric can extend down to an upper surface of a horizontal portion of the gate dielectric that overlies the global bit line, to a liner over the gate dielectric, or the global bit line itself depending on a particular configuration. Between horizontally adjacent pillars, this same dielectric layer extends from its same lower level beneath the gates vertically to a level of the upper source/drain region.

The vertical TFT select device is fabricated in one embodiment by first forming pillar stack lines that include a plurality of select device pillars. Each pillar includes a body, an upper source/drain region, and a lower source/drain region. Additional regions such as contact and insulating regions may be formed over the upper source/drain region. A gate dielectric is formed that extends along vertical sidewalls of the layer stack lines, followed by forming at least one gate for each pillar. In one example, each gate extends horizontally along the vertical sidewalls of a corresponding layer stack line. Each gate is recessed to create a void underlying the lower surface of the gate. A gap fill dielectric is formed after creating the void. The gap fill dielectric underlies the gate. The gap fill dielectric extends horizontally and vertically between adjacent layer stack lines.

Notably, the gap fill dielectric may be formed in a single process step as a continuous film that both underlies the gates, as well as extends vertically between gates of adjacent pillars. Such an approach may be contrasted with techniques that utilize two gap fill dielectric materials to insulate neighboring structures. For example, other approaches may form a first gap fill dielectric prior to forming the gates so that the lower endpoint of each gate is defined by forming the gate material over this pre-formed first dielectric. The first gap fill dielectric may be recessed using time etching to define an upper surface of the first dielectric that defines the lower endpoint of each gate. A second gap fill dielectric is then used to fill the spaces between adjacent gates and pillars after forming the gates in these techniques. Recessing the first dielectric may introduce variations in thickness of the first dielectric between different pillars, leading to differently sized gates as well. In accordance with the disclosed technology, a single gap fill dielectric material is formed after gate formation to reduce variations in the gate dimensions of different devices.

In one embodiment, the vertical TFT is used as a bit line selection device in a three-dimensional (3D) array of memory elements where bit lines of the array are oriented vertically. That is, instead of stacking a plurality of two-dimensional arrays on a common semiconductor substrate, where each two-dimensional array has its own bit lines, multiple two-dimensional arrays are stacked on top of each other in separate planes but then share common bit lines that extend up through the planes. As a selection device, the vertical TFT may be referred to as a select gate or decoder. Memory elements in a 3D memory array may be controlled by applying proper voltages to their vertical bit lines and word lines. By applying either a select voltage or an unselect voltage to the vertical bit lines, while applying either a select voltage or an unselect voltage to the horizontal word lines, memory cells are selected/unselected for operations such as SET, RESET, and FORM. The vertically TFT selection device provides the proper voltage to the vertical bit line.

In FIG. 1, an architecture of a three-dimensional memory 10 is described using a schematic of an equivalent circuit of a portion of the 3D memory. A standard three-dimensional rectangular coordinate system 11 is used for reference, the directions of each of vectors x, y and z being orthogonal with the other two. In another embodiment direction x and y are substantially 60 degrees from each other. The array in FIG. 1 includes vertical bit lines.

A circuit for selectively connecting internal memory elements with external data circuits is formed using select devices Q_(xy), where x gives a relative position of the device in the x-direction and y its relative position in the y-direction. The individual select devices Q_(xy) are vertical TFTs in accordance with embodiments. Global bit lines (GBL_(x)) are elongated in the y-direction and have relative positions in the x-direction that are indicated by the subscript. The global bit lines (GBL_(x)) are individually connectable with the source or drain of the vertical TFT select devices Q_(xy) having the same position in the x-direction, although during reading and also typically programming only one select device connected with a specific global bit line is turned on at time. The other of the source or drain of the individual select devices Q_(xy) is connected with one of the local bit lines (LBL_(xy)). The local bit lines are elongated vertically, in the z-direction, and form a regular two-dimensional array in the x (row) and y (column) directions.

In order to connect one set (in this example, designated as one row) of local bit lines with corresponding global bit lines, row select lines SG_(y) are elongated in the x-direction and connect with control terminals (gates) of a single row of vertical TFT select devices Q_(xy) having a common position in the y-direction. The vertical TFT select devices Q_(xy) therefore connect one row of local bit lines (LBL_(xy)) across the x-direction (having the same position in the y-direction) at a time to corresponding ones of the global bit-lines (GBL_(x)), depending upon which of the row select lines SG_(y) receives a voltage that turns on the vertical TFT select devices to which it is connected. The remaining row select lines receive voltages that keep their connected vertical TFT select devices Q_(xy) off. It may be noted that since only one vertical TFT select device (Q_(xy)) is used with each of the local bit lines (LBL_(xy)), the pitch of the array across the semiconductor substrate in both x and y-directions may be made very small, and thus the density of the memory storage elements large.

Memory elements M_(zxy) are formed in a plurality of planes positioned at different distances in the z-direction above a substrate (which may be below the pillar select layer). Two planes 1 and 2 are illustrated in FIG. 1 but there will typically be additional planes such as 4, 6, 8, 16, 32, or even more. In each plane at distance z, word lines WL_(zy) are elongated in the x-direction and spaced apart in the y-direction between the local bit-lines (LBL_(xy)). The word lines WL_(zy) of each plane individually cross adjacent two of the local bit-lines LBL_(xy) on either side of the word lines. The individual memory storage elements Mz_(xy) are connected between one local bit line LBL_(xy) and one word line WL_(zy) adjacent these individual crossings. An individual memory element M_(zxy) is therefore addressable by placing proper voltages on the local bit line LBL_(cy) and word line WL_(zy) between which the memory element is connected. The voltages are chosen to provide the electrical stimulus necessary to cause the state of the memory element to change from an existing state to the desired new state. After the device is first fabricated, voltages may be selected to provide the electrical stimulus necessary to “form” the memory element, which refers to lowering its resistance from a virgin state. The levels, duration and other characteristics of these voltages depend upon the material that is used for the memory elements.

Each “plane” of the three-dimensional memory structure is typically formed of at least two layers, one in which the conductive word lines WL_(zy) are positioned and another of a dielectric material that electrically isolates the planes from each other. Additional layers may also be present in each plane, depending for example on the structure of the memory elements M_(zxy). The planes are stacked on top of each other above a semiconductor substrate with the local bit lines LBL_(xy) being connected with storage elements M_(zxy) of each plane through which the local bit lines extend.

The memory arrays described herein, including memory 10, may be monolithic three dimensional memory arrays. A monolithic three dimensional memory array is one in which multiple memory levels are formed above (and not in) a single substrate, such as a wafer, with no intervening substrates. The layers forming one memory level are deposited or grown directly over the layers of an existing level or levels. In contrast, stacked memories have been constructed by forming memory levels on separate substrates and adhering the memory levels atop each other, as in Leedy, U.S. Pat. No. 5,915,167, “Three Dimensional Structure Memory.” The substrates may be thinned or removed from the memory levels before bonding, but as the memory levels are initially formed over separate substrates, such memories are not true monolithic three dimensional memory arrays.

The material used for the non-volatile memory elements M_(zxy) in the arrays described herein can be a chalcogenide, a metal oxide, CMO, or any one of a number of materials that exhibit a stable, reversible shift in resistance in response to an external voltage applied to or current passed through the material. Other materials can also be used. The technologies described below are not restricted to any one set of materials for forming the non-volatile memory elements.

By way of example, metal oxides are characterized by being insulating when initially deposited. One suitable metal oxide is a titanium oxide (TiO_(x)). A composite structure can be formed in a non-conductive (high resistance) state. When a large negative voltage (such as 1.5 volt) is applied across the structure, a relatively high current can flow through the structure. The device is then in its low resistance (conductive) state. The conductive path is broken by applying a large positive voltage across the structure. The device returns to its high resistance state. Both of the conductive and non-conductive states are non-volatile. Examples of other oxide materials that can be used for the non-volatile memory elements M_(zxy) in the array include HfOx, ZrOx, WOx, NiOx, CoOx, CoAlOx, MnOx, ZnMn₂O₄, ZnOx, TaOx, NbOx, HfSiOx, HfAlOx.

Another class of materials suitable for the memory storage elements includes solid electrolytes. They are electrically conductive when deposited. Individual memory elements can be formed and isolated from one another. Examples of solid electrolytes materials are: TaO, GeSe or GeS. Other systems suitable for use as solid electrolyte cells are: Cu/TaO/W, Ag/GeSe/W, Cu/GeSe/W, Cu/GeS/W, and Ag/GeS/W.

Carbon may also be used as a non-volatile memory element. Carbon is usually used in two forms, conductive (or graphite like-carbon) and insulating (or amorphous carbon). The operation of a carbon resistive switching nonvolatile memories involves transforming chemical bond configurations by applying appropriate current (or voltage) pulses to the carbon structure. For example, when a very short high amplitude voltage pulse is applied across the material, the conductance is greatly reduced as the carbon may be in an amorphous state. On the other hand, when in the reset state, applying a lower voltage for a longer time causes part of the material to change into the conductive state. Carbon nanotubes (CNTs) may be used as a non-volatile memory material. Such nanotubes can demonstrate very high conductivity. When an electric field is applied across this fabric, the CNT's tend to flex or align themselves such that the conductivity of their fabric is changed.

Yet another class of materials suitable for the memory storage elements is phase-change materials. A group of phase-change materials may include chalcogenide glasses, often of a composition Ge_(x)Sb_(y)Te_(z), where x=2, y=2 and z=5. GeSb may also be used. Other materials include AgInSbTe, GeTe, GaSb, BaSbTe, InSbTe and various other combinations of these basic elements. When a high energy pulse is applied for a very short time to cause a region of the material to melt, the material “quenches” in an amorphous state, which is a low conductive state. When a lower energy pulse is applied for a longer time such that the temperature remains above the crystallization temperature but below the melting temperature, the material crystallizes to form poly-crystal phases of high conductivity.

It will be noted that the memory materials in most of the foregoing examples utilize electrodes on either side thereof whose compositions are specifically selected. In embodiments of the three-dimensional memory array herein where the word lines (WL) and/or local bit lines (LBL) also form these electrodes by direct contact with the memory material, those lines can be made of the conductive materials described above. In embodiments using additional conductive segments for at least one of the two memory element electrodes, those segments can be made of the materials described above for the memory element electrodes.

Steering elements are commonly incorporated into controllable resistance types of memory storage elements. Steering elements can be a transistor or a diode. Although an advantage of the three-dimensional architecture described herein is that such steering elements are not necessary, there may be specific configurations where steering elements are included. The diode can be a p-n junction (not necessarily of silicon), a metal/insulator/insulator/metal (MIIM), or a Schottky type metal/semiconductor contact but can alternately be a solid electrolyte element.

For simplicity the above description has considered the simplest case of storing one data value within each cell: each cell is either reset or set and holds one bit of data. However, the techniques of the present application are not limited to this simple case. By using various values of ON resistance and designing the sense amplifiers to be able to discriminate between several of such values, each memory element can hold multiple-bits of data in a multiple-level cell (MLC).

Additional information regarding the various memory materials that may be used can be found in U.S. patent application Ser. No. 13/788,990, entitled Vertical Bit line TFT Decoder for High Voltage Operation,” filed Mar. 7, 2013 and incorporated by reference herein in its entirety.

FIG. 2 is a partial schematic of a three-dimensional memory 10 that includes a double-gated structure for the vertically oriented TFT select devices Qxy. Planes 1 and 2 of FIG. 2 are the same as in FIG. 1. As can be seen, each local bit line LBL is connectable to a respective global bit line GBL by two row select signals. FIG. 2 shows two transistors connecting to each local bit line. For example, transistor Q₁₁ can connect local bit line LBL₁₁ to global bit line GBL₁ in response to row select line SG₁ and transistor Q_(11a) can connect local bit line LBL₁₁ to global bit line GBL₁ in response to row select line SG₂. The same structure is used for the other local bit lines depicted in FIG. 2.

FIG. 3 shows another partial schematic also depicting the double-gated structure such that each local bit line (LBL1, LBL2, . . . LBL72) are connected to their respective global bit lines (GBL1, GBL2, . . . GBL72) by any of two respective vertically oriented TFT select devices that are positioned above a CMOS substrate. As can be seen, while the double-gated structure includes positioning the various select devices 504 above the substrate, the Row Select Line Drivers providing the row select lines SG₁, SG₂, . . . are positioned in the substrate. Similarly, the global word lines (e.g., GWL) are position in a metal layer on the substrate and below the vertically oriented select devices. Furthermore, the Row Select Line Driver uses the appropriate global word line GWL as an input in one embodiment.

FIG. 4 is a block diagram of an illustrative memory system that can use the three-dimensional memory 10 of FIGS. 1-3. Data input-output circuits 21 are connected to provide (during programming) and receive (during reading) analog electrical quantities in parallel over the global bit-lines GBL that are representative of data stored in addressed memory elements M_(zxy). Data input-output circuits 21 typically contain sense amplifiers for converting these electrical quantities into digital data values during reading, which digital values are then conveyed over lines 23 to a memory system controller 25. Conversely, data to be programmed into the array 10 are sent by the controller 25 to the input-output circuits 21, which then programs that data into addressed memory element by placing proper voltages on the global bit lines GBL_(x). For binary operation, one voltage level is typically placed on a global bit line to represent a binary “1” and another voltage level to represent a binary “0”. The memory elements are addressed for reading or programming by voltages placed on the word lines WL_(zy) and row select lines SG_(y) by respective word line select circuits 27 and local bit line circuits 29. In the specific three-dimensional array of FIGS. 1-3, the memory elements lying between a selected word line and any of the local bit lines LBL_(xy) connected at one instance through the select devices Q_(xy) to the global bit lines GBL may be addressed for programming or reading by appropriate voltages being applied through the select circuits 27 and 29.

Controller 25 typically receives data from and sends data to a host system 31. Controller 25 usually contains an amount of random-access-memory (RAM) 34 for temporarily storing such data and operating information. Commands, status signals and addresses of data being read or programmed are also exchanged between the controller 25 and host 31. The memory system operates with a wide variety of host systems. They include personal computers (PCs), laptop and other portable computers, cellular telephones, personal digital assistants (PDAs), digital still cameras, digital movie cameras and portable audio players. The host typically includes a built-in receptacle 33 for one or more types of memory cards or flash drives that accepts a mating memory system plug 35 of the memory system but some hosts require the use of adapters into which a memory card is plugged, and others require the use of cables therebetween. Alternatively, the memory system may be built into the host system as an integral part thereof.

Controller 25 conveys to decoder/driver circuits 37 commands received from the host 31. Similarly, status signals generated by the memory system are communicated to the controller 25 from decoder/driver circuits 37. The circuits 37 can be simple logic circuits in the case where the controller controls nearly all of the memory operations, or can include a state machine to control at least some of the repetitive memory operations necessary to carry out given commands. Control signals resulting from decoding commands are applied from the circuits 37 to the word line select circuits 27, local bit line select circuits 29 and data input-output circuits 21. Also connected to the circuits 27 and 29 are address lines 39 from the controller that carry physical addresses of memory elements to be accessed within the array 10 in order to carry out a command from the host. The physical addresses correspond to logical addresses received from the host system 31, the conversion being made by the controller 25 and/or the decoder/driver 37. As a result, the local bit line select circuits 29 partially address the designated storage elements within the array 10 by placing proper voltages on the control elements of the select devices Q_(xy) to connect selected local bit lines (LBL_(xy)) with the global bit lines (GBL_(x)). The addressing is completed by the circuits 27 applying proper voltages to the word lines WL_(zy) of the array.

Although each of the memory elements M_(zxy) in the array of FIGS. 1-3 may be individually addressed for changing its state according to incoming data or for reading its existing storage state, it may be preferable to program and read the array in units of multiple memory elements in parallel. One row of memory elements on one plane may be programmed and read in parallel, for example. The number of memory elements operated in parallel depends on the number of memory elements connected to the selected word line. In some arrays, the word lines may be segmented (not shown in FIGS. 1-3) so that only a portion of the total number of memory elements connected along their length may be addressed for parallel operation, namely the memory elements connected to a selected one of the segments. In some arrays the number of memory elements programmed in one operation may be less than the total number of memory elements connected to the selected word line to minimize IR drops, to minimize power, or for other reasons.

Previously programmed memory elements whose data have become obsolete may be addressed and re-programmed from the states in which they were previously programmed. The states of the memory elements being re-programmed in parallel will therefore most often have different starting states among them. This is acceptable for many memory element materials but it is may be preferred to re-set a group of memory elements to a common state before they are re-programmed. For this purpose, the memory elements may be grouped into blocks, where the memory elements of each block are simultaneously reset to a common state, preferably one of the programmed states, in preparation for subsequently programming them. If the memory element material being used is characterized by changing from a first to a second state in significantly less time than it takes to be changed from the second state back to the first state, then the reset operation is preferably chosen to cause the transition taking the longer time to be made. The programming is then done faster than resetting. The longer reset time is usually not a problem since resetting blocks of memory elements containing nothing but obsolete data is typically accomplished in a high percentage of the cases in the background, therefore not adversely impacting the programming performance of the memory system.

With the use of block re-setting of memory elements, a three-dimensional array of variable resistive memory elements may be operated in a manner similar to flash memory arrays. Resetting a block of memory elements to a common state corresponds to erasing a block of flash memory elements to an erased state. The individual blocks of memory elements may be further divided into a plurality of pages of storage elements. The memory elements of a page can be programmed and read together. This is similar to the use of pages in flash memories. The memory elements of an individual page are programmed and read together. When programming, those memory elements that are to store data that are represented by the reset state are not changed from the reset state. Those of the memory elements of a page that need to be changed to another state in order to represent the data being stored in them have their states changed by the programming operation. Example resetting, programming (e.g., setting) and reading operations of a memory array like that of FIGS. 1-3 can be found in U.S. patent application Ser. No. 13/788,990, entitled Vertical Bit line TFT Decoder for High Voltage Operation,” filed Mar. 7, 2013 and incorporated by reference herein in its entirety.

One example semiconductor structure for implementing a three-dimensional memory element array of FIG. 2 is illustrated in FIG. 5 which is configured for use of non-volatile memory (NVM) material that is non-conductive when first deposited. A metal oxide can be used for this characteristic in one example. Since the material is initially non-conductive, the memory elements at the cross-points of the word and bit lines are not isolated from each other. Several memory elements may be implemented by a single continuous layer of material, which in the case of FIG. 5 are strips of NVM material oriented vertically along opposite sides of the vertical bit lines in the y-direction and extending upwards through all the planes. A significant advantage of the structure of FIG. 5 is that all word lines and strips of insulation under them in a group of planes may be defined simultaneously by use of a single mask, thus greatly simplifying the manufacturing process.

A small part of four planes 101, 103, 105 and 107 of the three-dimensional array are shown. Elements of the FIG. 5 array that correspond to those of the equivalent circuit of FIG. 1 are identified by the same reference numbers. It will be noted that FIG. 5 shows the two planes 1 and 2 of FIG. 3 plus two additional planes on top of them. All of the planes have the same horizontal pattern of conductor, dielectric and NVM material. In each plane, metal word lines (WL) are elongated in the x-direction and spaced apart in the y-direction. Each plane includes a layer of insulating dielectric that isolates its word lines from the word lines of the plane below it or, in the case of plane 101, of the substrate circuit components below it. Extending through each plane is a collection of metal local bit line (LBL) “pillars” elongated in the vertical z-direction and forming a regular array in the x-y direction.

Each bit line pillar is connected to one of a set of global bit lines (GBL) running in the y-direction at the same pitch as the pillar spacing through the vertical TFT select devices (Q_(xy)) whose gates are driven by the row select lines (SG) elongated in the x-direction. The vertical TFT select devices have a channel extension, in one embodiment.

Not shown in FIG. 5 are sense amplifiers, input-output (I/O) circuitry, control circuitry, and other peripheral circuitry. There is one row select line (SG) for each row of local bit line pillars in the x-direction and two vertical TFT select device (Q) for each individual vertical local bit line (LBL).

Each vertical strip of NVM material is sandwiched between the vertical local bit lines (LBL) and a plurality of word lines (WL) vertically stacked in all the planes. Preferably the NVM material is present between the local bit lines (LBL) in the x-direction. A memory storage element (M) is located at each intersection of a word line (WL) and a local bit line (LBL). In the case of a metal oxide described above for the memory storage element material, a small region of the NVM material between an intersecting local bit line (LBL) and word line (WL) is controllably alternated between conductive (set) and non-conductive (reset) states by appropriate voltages applied to the intersecting lines. In one embodiment, the NVM material includes Hafnium Oxide, the word lines comprise TiN, and the bit lines comprise N+ silicon.

There may also be a parasitic NVM element formed between the LBL and the dielectric between planes. By choosing the thickness of the dielectric strips to be large compared to the thickness of the NVM material layer (that is, the spacing between the local bit lines and the word lines), a field caused by differing voltages between word lines in the same vertical word line stack can be made small enough so that the parasitic element does not conduct a significant amount of current. Similarly, in other embodiments, the non-conducting NVM material may be left in place between adjacent local bit lines if the operating voltages between the adjacent LBLs remain below the programming threshold.

To enable the memory to be denser (e.g., more memory elements per area), the size of the memory elements can be made small and the memory elements can be arranged close to each. To enable the memory elements to be close to each other, one embodiment uses a vertically oriented TFT decoder for connecting the individual local vertical bit line pillars to the respective global bit lines. In one embodiment, each vertically oriented TFT select device is a pillar select device that is formed as a vertical structure, switching between a local bit line pillar and a global bit line. The vertical TFT select devices, are in the present embodiments formed in a separate layer (pillar select layer) above the CMOS layer/substrate, along the z-direction between the array of global bit lines and the array of local bit lines. The CMOS layer is the substrate where the support circuitry is implemented, including the row select circuit and word line drivers. The use of vertically oriented TFT select devices above, but not in, the substrate allows the memory elements to be arranged in a more compact fashion, thereby increasing density. Additionally, positioning the vertically oriented TFT select devices above the substrate allows for other devices (e.g., the word line drivers) to be positioned in the substrate under the memory array rather than outside of the array, which allows the integrated circuit to be smaller.

For example, a pillar shaped vertical Thin Film Transistor (TFT) can be can be used as the select device. In one example implementation, a control node of the select transistor has a collar shaped hole, and the gate and channel region are formed in the hole with the source/drain regions formed above/below the channel region. Another alternative is to define the gates as a rail etch and have the channel deposited in a trench between the gates and singulated by an etch with crossing lines mask (rather than holes).

FIG. 6 illustrates the structure of a vertically oriented TFT select device in relation to the local bit line and the global bit line. The 3D memory 10 is formed on top of a CMOS substrate (not shown explicitly) where structures in the CMOS are referred to as being in the FEOL (“Front End of Lines”). The vertically oriented TFT select devices switch individual vertical bit lines (that are above and not in the substrate) to individual global bit lines that are formed on top of the FEOL layer in the BEOL (“Back End of Lines”). Thus, the BEOL includes the pillar select layer with the memory layer on top of it. The vertically oriented TFT select devices (such as Q₁₁, Q₁₂, . . . , Q₂₁, Q₂₂, . . . , etc.) are formed in the pillar select layer as vertically oriented TFT select devices. The pillar select layer is formed above (and not in) the substrate. The vertically oriented TFT select device may switch a local bit line to a global bit line. In the example, the local bit line LBL 530 is switchable to the global bit line GBL 526 by a vertically oriented TFT select transistor 504 such as Q₁₁. The gate of the TFT select transistor Q₁₁ is controllable by a signal exerted on a row select line SG₁ (not shown).

The global bit line such as GBL 526 is formed below the vertically oriented TFT select device, in the FEOL as part of the metal layer-1 or metal layer-2 502. The vertically oriented TFT select device is formed in the BEOL layer on top of the GBL 526 (and above, but not in, the substrate). The local bit line LBL 530, in the form of a pillar, is formed on top of the vertically oriented select device 504. In this way, the vertically oriented TFT select device 504 can switch the local bit line pillar LBL to the global bit line GBL.

FIG. 7 depicts an embodiment of a vertical TFT selection device 504 in accordance with an embodiment of the disclosure. FIG. 7 is a diagram of one embodiment depicting two vertical TFT selection devices 504 a and 504 b, which may be used for selecting vertically oriented bit lines (not shown). Each vertical TFT selection device 540 a, 540 b has two source/drain (S/D) regions. The source/drain regions are on either side of the body. The source/drain regions and body are polysilicon in one embodiment. The vertically oriented TFT select devices 504 a, 504 b has an upper junction between the body and upper S/D and a lower junction between the body and lower S/D.

In some embodiments, the body and the S/D regions are of opposite conductivity. For example, the body may be p− and each S/D N+. As another example, the body may be n− and each S/D P+. Thus, the TFT could be an enhancement NFET (N+ p− N+) or enhancement PFET (P+n−P+), as two examples. In some embodiments, the body and the S/D regions have the same conductivity. For example, the body may be p− and each S/D P+. As another example, the body may be n− and each S/D N+. Thus, the TFT could be a depletion type N+ n− N+ (NFET) or depletion type P+ p− P+ (PFET), as two examples. For depletion type devices, N+ n− N+ (NFET) and P+ p− P+ (PFET), the level of conductivity of each S/D may be much higher than that of the body. In one embodiment, the S/D net doping concentration is 3-4 orders of magnitude greater than the body net doping concentration.

Each of the vertical TFT selection devices 504 a, 504 b has two gates 507 and a dielectric material 505 separating each gate from the body and S/D regions. This dielectric may be referred to as a gate dielectric as it separates gates 507 from the body and S/D regions. In one embodiment, the gate dielectric 505 extends along the sides of the source/drain regions and the body or each selection device. The gate dielectric can be elongated in the x-direction along the vertical sidewalls of the pillar structure including the body and S/D regions.

The gate dielectric separates the gates from either the body or the source/drain regions, as the case may be. In this example, each gate 507 extends vertically from below the lower junction to above the upper junction. That is, the gates 507 are formed directly adjacent to the vertical extent of the body and directly adjacent to a portion of the vertical extent of both S/D regions. The lower source/drain is connected to a global bit line in this embodiment. The upper source/drain can be connected to a vertically oriented bit line (not depicted in FIG. 7).

Note that the vertical bit line material may serve as one of the electrodes of read-write memory elements. In some embodiments, the material of the vertical BL is N+ poly. For some types of memory cells, N+ poly serves as a good electrode choice (on the BL side) to achieve desired operation of a specific memory cell and memory array. For other memory cells with different material composition, P+ poly may be chosen as an electrode (on BL side) for desired operation of the memory cell and array. The P+ polysilicon work function is different from N+ polysilicon, and may be more suitable for the electrode material to enable the most efficient memory cell operation. In this latter case, a PFET TFT selection device may be preferable due to its source/drain being P+ (thus matching the vertical bit line). Accordingly, reference to the global bit line 526 or vertical bit line may include reference to such electrodes included with other bit line material such as metals. For example, global bit line 526 may refer to a metal global bit line alone, a semiconductor global bit line along, or a combination of the two materials, such as an N+ or P+ material overlying a metal line.

A gap fill dielectric 520 such as an oxide is formed between adjacent select devices as an insulating material. Under each gate 507, the gap fill dielectric 520 extends vertically from a lower surface or endpoint region of each gate 507 to an upper surface of the global bit line 526. In another example, the gate dielectric 505 may be formed over the global BL 526 such that the gap fill dielectric 520 extends from an upper surface of the gate dielectric to a lower surface of each gate.

Between adjacent gates 507, dielectric 520 extends vertically from an upper surface of global BL 526 (or gate dielectric 505) to at least the level of the upper surface of the upper source/drain region. Dielectric 520 is formed in a single process step to both fill the area or void beneath each gate, as well as to fill the areas between adjacent gates. A single gap fill process is used in one embodiment to fill both of the spaces after formation of the gates 507 is complete. Accordingly, the gap fill dielectric is a continuous film that extends under gates 507 as well as between the gates of adjacent select devices 504.

By forming a single gap fill dielectric after the gates, traditional oxide recess processes to form a first gap fill material before gate formation can be avoided. For example, traditional processes may deposit and recess a first oxide prior to forming gates 507. After recessing the oxide, the gate material can be deposited over the first oxide. In this manner, the amount of recess of the oxide controls the lower surface or endpoint of each gate. In order to recess an oxide, fixed time etching may be used, as end point detection mechanisms may not be available. Timed etching to recess the oxide introduces variation in the amount the oxide is recessed, which in turn leads to varying dimension of gates 507. A second oxide in these traditional processes may be formed after gate formation. Thus, a first oxide will be formed under each gate in a first gap fill process and a second oxide will be formed between adjacent gates in a second gap fill process.

In FIG. 7, however, dielectric 520 is formed in a single process step after forming gates 507. This oxide is formed after the lower surface or endpoint of the gates has been defined. An oxide recess step is not required for gate formation, nor is a two-step process required for gap fill formation. Accordingly, variations in the lower endpoint of the gates can be avoided or minimized when compared with traditional processes. Moreover, a single step process is used for gap fill formation, leading to cost and efficiency improvements.

FIG. 8 is a cross-sectional view of a memory structure using a vertically oriented TFT select device 504 as shown in FIG. 7 with the memory structure of FIG. 5. The TFT select devices 504 are each separated by gap fill material formed in a single process step after gate formation. The gate of the TFT select devices are separated vertically from the underlying global bit line, at least partially by the gap fill material which is formed after the gate and bit line. FIG. 8 shows ten gates 507 in a gate material layer, each underneath a stack of multiple word lines. As can be seen, two gates are positioned between pairs of vertically oriented select devices 504, above and not in the substrate. Each select device includes two gates, one on each side formed along a vertical sidewall of the pillar structure. The gates 507 extend horizontally in the x-direction (not shown) and can form row select lines SG_(x) as shown in FIGS. 1-5.

The memory layer includes a set of vertical bit lines 530 (comprising N+ polysilicon). Interspersed between the vertical bit lines 530 are alternating oxide layers 534 and word line layers 536. In one embodiment, the word lines are made from TiN. Between the vertical bit lines 530 and the stacks of alternating oxide layers 536 and word line layers 536 are vertically oriented layers of reversible resistance switching material 532. In one embodiment the reversible resistance switching material is made of Hafnium Oxide HfO₂. However, other materials (as described above) can also be used. Box 540 depicts one example memory element which includes the reversible resistance switching material 532 sandwiched between a word line 536 and vertical bit line 530. The memory elements are positioned above, and not in, the substrate.

Directly below each vertical bit line 530 are the vertically oriented TFT select devices 504, each of which comprises (in one example embodiment) an n+/p−/n+ TFT. Each n+ region may be referred to as a source/drain. The p− region may be referred to as a body. The p− region may serve as the channel of the TFT during operation. As noted above, other conductivities can be used in the TFT selection device, with suitable changes to other elements in the memory array.

Each of the vertically oriented TFT select devices 504 includes a gate dielectric 505 on each side. The dielectric 505 is oxide in one embodiment. In this embodiment, each TFT has two gates. Referring to TFT 504 a, there is a gate 507 ₁ to the left and a gate 507 ₂ to the right. The gate dielectric extends horizontally between adjacent select devices over the upper surface of the underlying global bit line 526 in this example.

In FIG. 8, the channel is not extended on the top of the TFT 504 or on the bottom of the TFT 504. That is, the p− region of each TFT 504 does not extend above the upper surface or portion of gate material 522, nor does the p− region of each TFT 504 extend below the lower surface of portion of gate material 522. However, the vertical TFT selection device 504 may have a channel extension in other embodiments. The channel extension may be formed either at the upper portion or lower portion. The body region extends past the gates in such an embodiment. The channel may be extended on the top or the bottom of the TFT 504. That is, the body region does not extend past the gates at either the lower junction or the upper junction. Thus, the gates are offset from the upper junction or lower junction in this embodiment. By the gate being offset from the junction, it is meant that that the gate is not directly adjacent to the junction. For example, the gates may not be directly adjacent to the upper junction or not directly adjacent to the lower junction. Another way of stating the foregoing is that the top portion of the gates does not extend above the upper junction or the bottom portion of the gates does not extend below the lower junction. In the case the top or/and bottom portion of the gates extend beyond the junction, the structure has respective gate junction overlap, or gate-source or/and gate/drain overlap. More detail regarding structures with a channel offset can be found in U.S. application Ser. No. 13/788,990, referenced above.

In FIG. 8, the dielectric 520 is a gap fill dielectric formed as a single layer that extends vertically beneath gates 507 and horizontally between gates 507 of adjacent select device pillars. For example, dielectric 520 extends vertically between a lower surface of gate 507 ₂ of select device 504 a and an upper surface of gate dielectric 505 that overlies the global bit line 526. Further, layer 520 extends horizontally between gate 507 ₂ of select device 504 a and gate 507 ₁ of select device 504 b. Dielectric 520 extends vertically between each adjacent pair of gates and over each gate to the upper surface of the uppers S/D region.

Notably, dielectric 520 is not etched back or recessed between the adjacent select devices. Dielectric 520 extends continuously as a single layer vertically in the voids between adjacent select devices. A single material underlies the gates 507, as well as fills the spaces between adjacent gates. Moreover, this material extends vertically from a level adjacent the lower n+ S/D region to an upper surface of the upper n+ S/D region. The gap fill dielectric 520 may extend vertically above the upper surface of the upper S/D region in another example. The formation of a single layer without recess enables a more uniform formation of gates 507. Gates 507 are formed prior to formation of gap fill material 520 in a process that does not rely on gap fill or recess to define a gate dimension. In this manner, variations in the amount of recess of a gap fill material which may affect the dimensions of the gates are avoided or minimized.

The memory structure of FIG. 8 is a continuous mesh array of memory elements because there are memory elements connected to both sides of the bit lines and memory elements connected to both sides of the word lines. At the bottom of FIG. 8, the CMOS substrate is depicted. Implemented on the top surface of the CMOS structure are various metal lines including ML-0, ML-1, and ML-2. Line 526 of ML-2 serves as a respective global bit line (GBL). The metal line ML-2 526 serving as a global bit line can be implemented of any suitable material, including Tungsten, or Tungsten on a Titanium Nitride adhesion layer or a sandwich of n+ polysilicon on Tungsten on Titanium Nitride adhesion layer. Gate material 522 can be polysilicon, Titanium Nitride, Tantalum Nitride, Nickel Silicide or any other suitable material. Gate material 522 implements the row select lines SG_(x) (e.g. SG₁, SG₂, . . . of FIG. 1), which are labeled in FIG. 8 as row select lines 507. Portions of the row select lines may also be referred to a transistor gates. In one embodiment, an N+ line overlies metal ML-2 serving as part of the global bit line.

FIG. 9 is a flowchart describing a method of fabricating a pillar select layer having a vertical TFT select device 504 in one embodiment. The process of FIG. 9 may be used to form the pillar select layer depicted in FIGS. 7 and 8. In this embodiment, a single gap fill dielectric is formed after gate formation to facilitate uniformity in the select devices and provide efficiency and lower cost for the fabrication.

At step 604, pillar stack lines for a pillar select layer are formed over a metal layer. The pillar stack lines are formed in a direction orthogonal to a direction of the global bit lines in one example. FIGS. 10a and 10b depict the results of step 604 in one embodiment. FIG. 10a is a perspective view depicting four pillar stack lines 780, each containing two select gate (SG) or select device pillars 782. FIG. 10b is a cross-sectional view in the y-direction depicting a select gate pillar from each of the pillar select lines that are depicted in FIG. 10a . It is noted that a typical memory will include many more pillar select lines and within each pillar select line, many more select gate pillars. Step 604 and the later steps may be performed before manufacturing the memory layer. The substrate layers and metal layers can be manufactured, followed by forming the select gate layer stack lines for the pillar select structure. The metal layers include a metal layer (e.g., metal layer ML-2) that is patterned and formed into global bit lines extending in the y-direction.

In this example, the metal layer ML-2 has been patterned and etched into global bit lines that are elongated in the y-direction. The global bit lines are separated by a dielectric 716 such as an oxide or other insulating material. Over the global bit lines 526 and dielectric 716 are formed pillar stack lines 780 that are elongated in the x-direction. Each pillar stack line includes a plurality of select gate pillars 782. Each pillar is separated from adjacent pillars in the same line by insulating material 716. In another example, the insulating material between bit lines and the insulating material between select gate pillars may be different.

Pillar stack lines 780 are formed in one example by forming a layer stack that includes the layers of the select gate pillars. The layer stack may also include metal layer ML-2 in one example so that it may be etched using the same pattern for self-alignment. The layer stack can be etched first to define lines that extend in the y-direction. This etching may further define the global bit lines 526. After forming lines that extend in the y-direction, insulating material 716 may be deposited to fill the spaces between lines adjacent in the x-direction. After forming the insulating material, the layer stack may be patterned and etched to form lines 780 that extend in the x-direction over the global bit lines and insulating material 716. In this manner, select gate pillars 782 are formed in each line 780 that are separated from adjacent pillars of the line by pillars of insulating material 716. It is noted that other techniques may be used for forming select gate pillars 782 and pillar stack lines 780.

Each select gate pillar includes a lower n+ region forming the lower S/D region for the select device, a p− region forming the body, and an upper n+ region forming the upper S/D region. Note that the upper S/D region is a drain in one example and the lower S/D region is a source. In other example, the upper S/D region is a source and the lower S/D region is a drain. As described above, different configurations and materials may be used to form the body and S/D regions. Each pillar in FIG. 10a also includes a metal region 712 and a dielectric region 714. The metal region 712 and dielectric region 714 may be used to form a contact to the overlying vertical metal bit line, for example. For example metal region 712 is TiN in one embodiment and dielectric region 714 is SiN. These regions are optional and are not included in other embodiments.

At step 606, the gate dielectric is formed. FIG. 10c depicts the results of step 606 in one example. The metal layer an CMOS layer depictions have been removed from FIG. 10c for conciseness. Gate dielectric 505 extends over the upper surface and along the vertical sidewalls (elongated in the x-direction) of each select gate pillar. In one example, gate dielectric 505 is an oxide such as silicon oxide or hafnium oxide formed by atomic layer deposition (ALD). In another example, gate dielectric 505 is a thermally grown oxide. Conformal deposition can be used to form a dielectric layer between approximately 3 and 10 nanometers in thickness in one example. Other materials, processes and dimensions may be used. Chemical vapor deposition (CVD) processes, metal organic CVD processes, physical vapor deposition (PVD) processes, atomic layer deposition (ALD) processes, or other suitable techniques can be used to form the various layers described herein except where otherwise noted.

At step 608, the gate material is formed. FIG. 10d depicts the results of step 608 in one example. Gate material 717 is formed similarly to gate dielectric material 505. The gate material can be deposited conformally to cover the gate dielectric 505 in one example. The gate material covers the gate dielectric, extending vertically with the gate dielectric along the vertical sidewalls of the select gate pillars and over the upper surface of the gate dielectric at the top of each pillar. In one embodiment, the gate material is TiN. In another embodiment, the gate material is polysilicon. In other example, any of the memory materials described above may be used.

At step 610, a sacrificial material is formed. FIG. 10e depicts the results of step 610 in one example. The sacrificial material 730 is a layer of SiN in one embodiment, and can be formed by a conformal deposition process to line gate material 717. The sacrificial material is formed along the vertical sidewalls of the gate material and over the upper surface of the gate material at the top of each select gate pillar. It is noted that the sacrificial material is formed prior to any etch or processing of the gate material 717.

At step 612, a recess process is performed to etch, polish or otherwise vertically recess the horizontally exposed surfaces. Step 612 includes recessing the pillar select lines at their horizontal upper surface as well as recessing the material between pillar select lines at their horizontally exposed surfaces. FIG. 10f depicts the results of step 612 in one example. Step 612 includes isotropic etching, such as wet etching or chemical dry etching in one example, although any suitable etch process may be used. Step 612 includes removing the bottom gate material between adjacent pillar stack lines as well as the gate material overlying each pillar stack line, while the vertical portions of the gate material remain. In one embodiment, step 612 include recessing or etching back any exposed horizontally extending surfaces in one example as shown in FIG. 10f . That is, the horizontally extending layers at the top of each pillar stack line and the horizontally extending layers between each pillar stack line are recessed. This includes removing material having an exposed upper surface while not removing or substantially removing material having an exposed vertical surface. In this manner, the upper portion of the pillar stack line is recessed until the gate dielectric is reached. Chemical mechanical polishing (CMP) may be used in one embodiment. For example, CMP followed by an additional etch technique for the removal of the bottom portion of the material between pillar stack lines.

Step 612 includes removing the horizontally exposed portions of the sacrificial material and the gate material at the top of each layer stack line. Additionally, the sacrificial liner material and the gate material 717 are removed where the horizontally extending upper surface of the sacrificial liner material was exposed between layer stack lines. In one example, the gate dielectric 505 remains between the layer stack lines and at the upper portion of each layer stack line. In another example (not shown), the gate dielectric material may be removed from these portions as well at step 612. Recessing the gate material 717 between layer stack lines forms individual gates 507 that extend vertically along the vertical sidewalls of the layer stack lines, and are separated therefrom by the gate dielectric 505. In this example, gates 507 may be referred to as spacer gates 507 due to their formation by conformal deposition and etch back as associated with spacer-formation processes.

At step 614, the gates 507 are recessed or etched back to reduce the vertical dimension of each gate 507. FIG. 10g depicts the results of step 614 in one example. Part of the upper portion and lower portion of each gate 507 is removed. As shown in FIG. 10f , the initial recess process exposes an upper surface of each gate, as well as a lower portion of the vertical sidewall of each gate. Step 614 may include etching preferentially for the gate material (e.g., polysilicon) relative to the sacrificial liner (e.g., SiN). In this manner, etching will remove the gate material at its exposed upper surface and its exposed lower portion of its vertical sidewall. Different amounts of each gate 507 may be removed in step 614 in different implementations. Etching back the gate 507 at the lower portion of its vertical sidewall creates voids 740 beneath each gate 507. That is, a void exists between the lower surface of each gate and the upper surface of the gate dielectric that overlies the global bit line. It is noted that the portions of the gate dielectric material underlying the gates may be removed so that the voids extend between the lower surface of each gate and the upper surface of the global bit line. In FIG. 10g , the upper surface of each gate 507 is recessed to a level above the junction between the body and upper n+ S/D region. In another example, the upper surface of each gate may be recessed to a level below this junction to create a channel offset. The lower surface of each gate 507 is etched back to a level below the junction between the body and the lower n+ S/D region. In another example, the lower surface of each gate may be recessed to a level above the junction to create a channel offset.

At step 616, the remaining portions of the sacrificial material are removed. FIG. 10h depicts the results of step 616. In one example, an isotropic etch is used to remove SiN, selectively with respect to the gate material. Chemical dry etching, wet etching, or combinational dry/wet etch chemistries can be used.

At step 618, a top liner is formed. FIG. 10i depicts the results of step 618 in one example. A conformal deposition process is used to form a SiN liner 732 in one embodiment. Other materials and processes may be used. Liner 732 is formed along the vertical sidewalls of the gates 707 and the vertical sidewalls of the exposed portions of gate dielectric 705. Moreover, the liner 732 extends over the upper surface of the exposed horizontal extents of the gate dielectric 705.

At step 620, a gap fill dielectric is formed. FIG. 10j depicts the results of step 620 in one example. The gap fill dielectric 520 fills the spaces between adjacent layer stack lines. The gap fill dielectric 520 is an oxide formed using a flowable CVD gap fill technology scheme in one example. Polysilazane-based spin-on-glass techniques may be used. Notably, the entirety of the space between layer stack lines is filled with a single gap fill dielectric. Moreover, this dielectric 520 is formed subsequent to forming gates 507. In this manner, dielectric 520 fills the voids created beneath each gate 507 as a result of the recess at step 614. The dielectric extends vertically from the lower surface of each gate to the upper surface of the liner 732 that is formed over the gate dielectric 505. Vertically, the gap fill dielectric also extends from the upper surface of the liner to the upper surface of each layer stack line. In this example, the gap fill dielectric extends above each layer stack line, including over the upper surface of the gate dielectric overlying the SiN at the top of each layer stack line. Horizontally, the gap fill dielectric extends in the y-direction between adjacent layer stack lines. The dielectric 520 extends between the vertical sidewalls of the top liner that is formed along the gates and the exposed portions of the gate dielectric.

Although not shown, further processing may be performed to etch back or polish (e.g., CMP) the gap fill dielectric until reaching liner 732. In another example, processing etches back to expose the upper S/D region. Processing to form the memory layer, including vertical bit lines coupled to each of the select devices can be performed.

Accordingly, there has been described in one embodiment a non-volatile storage system that comprises a global bit line, a first vertical thin film transistor (TFT) select device above a substrate and coupled to the global bit line, and a second vertical thin film transistor (TFT) select device above the substrate and coupled to the global bit line. The first TFT select device includes a first gate that is separated from a first body by a gate dielectric. The first body extends vertically relative to the substrate. The first gate has a lower surface. The second TFT select device is horizontally adjacent to the first vertical TFT select device and includes a second TFT select device including a second gate that is separated from a second body by the gate dielectric. The second body extends vertically relative the substrate and the second gate has a lower surface. The system includes a gap fill dielectric formed between the first vertical TFT select device and the second vertical TFT select device. The gap fill dielectric underlies the lower surface of the first gate and the lower surface of the second gate. The gap fill dielectric extends horizontally at least partially between the first gate and the second gate. The gap fill dielectric beneath and between gates is a continuous film of dielectric material.

A method of forming a vertical thin film transistor (TFT) select device in one embodiment has been described that includes forming a body, a first source/drain (S/D) region and a second S/D region over a substrate. The first S/D region is coupled to a vertical bit line. The second S/D region is coupled to a global bit line. The method includes forming a gate dielectric after forming the body, the first S/D region, and the second S/D region, forming a gate after forming the gate dielectric that is separated from the body by the gate dielectric and has a lower surface that is separated from the global bit line at least partially by a void. The method includes forming a gap fill dielectric after forming the spacer gate. The gap fill dielectric is formed in the void such that the lower surface of the gate is separated from the global bit line by the gap fill dielectric and the gate dielectric.

A method of forming a vertical thin film transistor select device in one embodiment includes forming a select gate pillar above a substrate. The select gate pillar includes a body, a first source/drain and a second source/drain. The first source/drain is coupled to an upper surface of the body and a vertical bit line. The second source/drain is coupled to a lower surface of the body and a global bit line. The select gate pillar includes first and second vertical sidewalls elongated in a first direction. The method includes forming a gate dielectric along the first and the second vertical sidewalls and over an upper surface of the select gate pillar, forming a gate material separated from the select gate pillar by the gate dielectric, forming a sacrificial material along vertical sidewalls of the of the gate material and an upper surface of the gate material, and recessing the gate material and sacrificial material to form a first gate that is separated from the first vertical sidewall of the select gate pillar by the gate dielectric and a second gate that is separated from the second vertical sidewall of the select gate pillar by the gate dielectric. Recessing the liner exposes a lower portion of the first gate and a lower portion of the second gate. The method includes removing the lower portion of the first gate and the lower portion of the second gate and after removing the lower portion of the first gate and the lower portion of the second gate, forming a gap fill dielectric. The gap fill dielectric underlies a lower surface of the first gate and the second gate.

The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the subject matter claimed herein to the precise form(s) disclosed. Many modifications and variations are possible in light of the above teachings. The described embodiments were chosen in order to best explain the principles of the disclosed technology and its practical application to thereby enable others skilled in the art to best utilize the technology in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto. 

What is claimed is:
 1. A method of forming a vertical thin film transistor (TFT) select device, comprising: forming a first vertical TFT select device including a body, a first source/drain (S/D) region and a second S/D region over a substrate, the first S/D region coupled to a first vertical bit line, the second S/D region coupled to a global bit line; forming a second vertical TFT select device including a body, a first source/drain (S/D) region and a second S/D region over a substrate, the first S/D region coupled to a second vertical bit line, the second S/D region coupled to the global bit line; forming a gate dielectric after forming the body, the first S/D region, and the second S/D region of the first vertical TFT select device and the second vertical TFT select device; etching to form a first gate for the first vertical TFT select device and a second gate for the second vertical TFT select device after forming the gate dielectric, the first gate is separated from the body of the first vertical TFT select device by the gate dielectric and has a lower surface that is separated from the global bit line at least partially by a void, the second gate is separated from the body of the second vertical TFT select device by the gate dielectric and has a lower surface that is separated from the global bit line at least partially by a void; and forming a gap fill dielectric after forming the gate, the gap fill dielectric being formed in the void such that the lower surface of the gate is separated from the global bit line by the gap fill dielectric; wherein after etching to form the first gate and the second gate, the gate dielectric extends continuously in a horizontal direction over a portion of the global bit line between the second S/D region of the first TFT select device and the second S/D region of the second TFT select device.
 2. The method of claim 1, wherein: the gate dielectric extends vertically along a vertical sidewall of the body, the first S/D region, and the second S/D region; and the lower surface of the gate is separated from the global bit line by the gap fill dielectric and the gate dielectric.
 3. The method of claim 2, wherein forming the gate comprises: depositing a gate material over the gate dielectric; depositing a sacrificial material over the gate material; recessing the gate material and the sacrificial material to remove a portion of the sacrificial material and a portion of the gate material that overlie the gate dielectric where it extends horizontally, wherein recessing the gate material forms the gate; and removing a lower portion of the gate material that is exposed as a result of recessing the gate material and the sacrificial material, wherein removing the lower portion of the gate material forms the void.
 4. The method of claim 3, further comprising: removing the sacrificial material after removing the lower portion of the gate material; and forming a liner material after removing the sacrificial material, the liner material being formed along vertical sidewalls of the gate and over an upper surface of a portion of the gate dielectric that extends horizontally.
 5. The method of claim 4, wherein forming the gap fill dielectric comprises: depositing the gap fill dielectric using flowable chemical vapor deposition.
 6. The method of claim 1, wherein: the lower surface of the gate is separated from the global bit line by the void, a liner, and the portion of the gate dielectric formed over the global bit line.
 7. The method of claim 1, wherein: forming the gap fill dielectric includes at least partially filling a space between the gate of the vertical select device and a gate of an adjacent vertical TFT select device.
 8. The method of claim 7, wherein the gap fill dielectric extends vertically from above a horizontal portion of the gate dielectric that overlies the global bit line to at least a level of an upper surface of the body.
 9. The method of claim 1, wherein the first S/D region is coupled to the vertical bit line through at least one additional layer.
 10. The method of claim 1, wherein: the gate is a first spacer gate that is formed horizontally adjacent to a first vertical sidewall of the body; and the vertical TFT select device includes a second spacer gate that is formed horizontally adjacent to a second vertical sidewall of the body.
 11. A method of forming a vertical thin film transistor (TFT) select device, comprising: forming a select gate pillar above a substrate, the select gate pillar including a body, a first source/drain and a second source/drain, the first source/drain coupled to an upper surface of the body and a vertical bit line, the second source/drain coupled to a lower surface of the body and a global bit line, the select gate pillar including first and second vertical sidewalls elongated in a first direction; forming a gate dielectric along the first and the second vertical sidewalls and over an upper surface of the select gate pillar; forming a gate material separated from the select gate pillar by the gate dielectric; forming a sacrificial material along vertical sidewalls of the gate material and an upper surface of the gate material; recessing the gate material and sacrificial material to form a first gate that is separated from the first vertical sidewall of the select gate pillar by the gate dielectric and a second gate that is separated from the second vertical sidewall of the select gate pillar by the gate dielectric, wherein recessing the sacrificial material exposes a vertical sidewall of a lower portion of the first gate and a vertical sidewall of a lower portion of the second gate; removing the lower portion of the first gate and the lower portion of the second gate; and after removing the lower portion of the first gate and the lower portion of the second gate, forming a gap fill dielectric, the gap fill dielectric underlying a lower surface of the first gate and the second gate.
 12. The method of claim 11, wherein the select gate pillar is a first select gate pillar, the method further comprising: forming a pillar stack line including the first select gate pillar and a plurality of additional select gate pillars, each additional select gate pillar being coupled to one of a plurality of additional global bit lines.
 13. The method of claim 11, wherein: removing the lower portion of the first gate defines a first void between a lower surface of the first gate and the global bit line; removing the lower portion of the second gate defines a second void between a lower surface of the second gate and the global bit line; and forming the gap fill dielectric includes filling the first void and the second void.
 14. The method of claim 11, further comprising: forming a three-dimensional memory array of memory cells above a substrate; forming a plurality of word lines coupled to the memory cells; forming a plurality of global bit lines; forming a plurality of vertical bit lines coupled to the memory cells; and forming a plurality of vertical thin film transistor (TFT) select devices above the substrate, the vertical TFT select devices are coupled between the vertical bit lines and the global bit lines. 